Attic stair insulation is insulation which is designed for attic stairs. Attic stairs can be a major source of heat loss for a home, and they can cause other problems as well. Insulation is designed to create a tight seal so that these problems are eliminated. The insulation can be built into the unit, or it may take the form of a box or tent installed over the stair unit. Attic stair insulation is usually paired with weatherstripping which seals the edges of the attic stairs when they are in a closed position.
The obvious problem with attic stairs is that they effectively create a hole in the floor covered with a thin sheet of plywood. Heat from the house can be lost through this hole, as can cold air from air conditioning units in the summer, with the air ending up in the attic, where it doesn't do anybody any good. Furthermore, moist air can get into the attic through the attic stairs and become trapped, leading to mold and mildew on items stored in the attic as well as rot in the structural supports of the attic.
An opening for attic stairs can also create an ideal conduit for allergens from the attic. In houses where people have allergies, this can become a serious problem as materials like dust filter down through the attic. For all of these reasons, insulating attic stairs is a very good idea.
Some attic stair units address the insulation issue by building an insulated compartment into the stairs. This can make them a bit bulky, but can resolve the issues associated with attic stairs very well. In other cases, attic stair insulation is accomplished by creating a box in the attic which covers the attic stairs. When people climb the stairs to access the attic, they can push on the lid of the box to get into the attic. Even a partial covering is better than nothing, with people ducking through the opening in the box to get into the attic.
Several companies manufacture attic stair insulation of this type in units which people can simply install over the stairs. For people who prefer to do work of this kind on their own, it's very easy to build a box. Even a simple box made from stiff pieces of insulating foam will be beneficial, and no fancy features are required. It can also help to line the outside of the attic stair insulation with reflective material, to push heat from the attic away from the insulation.